Problem: A circle with circumference $20\pi$ has an arc with a $\dfrac{7}{9}\pi$ radians central angle. What is the length of the arc? ${20\pi}$ ${\dfrac{7}{9}\pi}$ $\color{#DF0030}{\dfrac{70}{9}\pi}$
Solution: The ratio between the arc's central angle $\theta$ and $2 \pi$ radians is equal to the the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{2 \pi} = \dfrac{s}{c}$ $\dfrac{7}{9}\pi \div 2 \pi = \dfrac{s}{20\pi}$ $\dfrac{7}{18} = \dfrac{s}{20\pi}$ $\dfrac{7}{18} \times 20\pi = s$ $\dfrac{70}{9}\pi = s$